fix(core): stringify shouldn't throw when toString returns null/undefined (#14975)

Fixes #14948

PR Close #14975

packages/core/src/util.ts

@@ -69,6 +69,11 @@ export function stringify(token: any): string {
   }
 
   const res = token.toString();
+
+  if (res == null) {
+    return '' + res;
+  }
+
   const newLineIndex = res.indexOf('\n');
   return newLineIndex === -1 ? res : res.substring(0, newLineIndex);
 }

packages/core/test/util_spec.ts

@@ -0,0 +1,19 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {stringify} from '../src/util';
+
+export function main() {
+  describe('stringify', () => {
+    it('should return string undefined when toString returns undefined',
+       () => expect(stringify({toString: (): string => undefined})).toBe('undefined'));
+
+    it('should return string null when toString returns null',
+       () => expect(stringify({toString: (): string => null})).toBe('null'));
+  });
+}